Elastomeric corrosion-inhibiting coating and sealing material

ABSTRACT

A PROTECTIVE COATING AND SEALING MATERIAL PARTICULARLY ADAPTED FOR FASTNERS, RIVETS AND OTHER METALLIC STRUCTURAL PARTS WHICH ARE IN INTIMATE CONTACT IS COMPOSED OF A RESILIENT POLYMER CONTAINING DISPERSED PARTICLES OF WATERSOLUBLE INHIBITOR. THE POLYMER WHEN CURED FORRMS A RESILIENT SEAL WHICH PREVENTS CORROSION FLUIDS, NOTABLY WATER, FROM ENTERING THE SPACE BETWEEN THE ADJACENT PARTS, BE IT A FASTENER AND ADJACENT HOLD SURFACES OR OVERLAPPING SHEET SURFACE. IF MOISTURE DOES DIFFUSE THROUGH THE POLYMER, SOME OF THE INHIBITOR IS DISSOLVED AND CONVERTS THE MOISTURE TO A CORROSION-PREVENTIVE SOLUTION. THIS MATERIAL MAY ALSO BE USED AS A DISCREET LAYER IN A COATING SYSTEM ON A METAL SURFACE WITH OR WITHOUT A TOP COAT, WHICH TOP COAT PREVENTS EXCESSIVE DIFFUSION OF MOISTURE INTO THE INHIBITED MATERIAL.

United States Patent US. Cl. 260-37 R 6 Claims ABSTRACT OF THEDISCLOSURE A protective coating and sealing material particularlyadapted for fasteners, rivets and other metallic structural parts whichare in intimate contact is composed of a resilent polymer containingdispersed particles of watersoluble inhibitor. The polymer when curedforms a resilient seal which prevents corrosion fluids, notably water,from entering the space between the adjacent parts, be it a fastener andadjacent hole surfaces or overlapping sheet surface. If moisture doesdiffuse through the polymer, some of the inhibitor is dissolved andconverts the mois ture to a corrosion-preventive solution. This materialmay also be used as a discreet layer in a coating system on a metalsurface with or without a top coat, which top coat prevents excessivediffusion of moisture into the inhibited material.

This application is a continuation-in-part of United States patentapplication Ser. No. 615,311 filed Feb. 13, 1967, and now abandoned, inthe name of the same inventors and assigned to the same assignee asherein, said prior application being abandoned concurrently with thefiling of this application.

This invention relates to corrosion-inhibting materials employed in theprotection of metal, and more particularly to improvements in suchmaterials for use as coatings and sealants offering especial utility inthe prevention of stress corrosion or exfoliation type corrosion in thearea of adjacent metal surfaces and/or fasteners when installed in theirrespective holes in metal. When used in a coating system, this materialprevents general corrosion and filiform corrosion of parts exposed tohumidty, salt-laden air and other corrosive environments, even seawater. Filiform corrosion, so-called because of its propagation underfilms (e.g. paints), as filaments, causes metal tocorrode, the coveringfilm to blister and finally fail but only after it has seriouslydegraded the metal structure.

While the present material may be employed to advantage wherevercorrosion is a problem, one of its princpal applications as presentlycontemplated resides in aircraft as a protective sealant between fayingsurfaces and between fasteners and the holes in the external surfaces orskins of the aircraft in which such fasteners are installed; and as aprimer for flexible coatings on metal parts and surfaces. Corrosioncaused by water entering aircraft or like structures through seams orcrevices particularly around skin fasteners is probably the most seriouscontributor to a shortened service life of aircraft. Since entry ofwater is possible by capillary flow, gravity is not a factor, and watercan flow in all directions and penetrate the tightest joints.

In an effort to solve these and related problems and thereby improve theabove corrosion conditions, various schemes have been devised. In one ofthe more popular practices, for example, fasteners are installedfollowing an injection of wet sealer, such as zinc chromate primer, intoa fastener hole. For large production use, this necessitates elaborateinstallation equipment to ensure the Patented May 1, 1973 properassembly and sequence. Also, cleanup problems are introduced both withrespect to time and the quality of the ultimate product. Attempts toovercome the objections to wet riveting as it is commonly called haveled to proposals whereby encapsulated zinc chromate liquid is applied tothe surface of the rivet. On installation, these capsules are ruptured,and the chromate liquid wets the adjacent rivet and hole surfaces. Thistends to eliminate much installation equipment, but entails added time,difficulty, and expense in the fabrication of fasteners. In addition,this coating system has a limited storage stability, Le, a relativelyshort shelf life.

Coated metal parts such as fasteners have more recently been suggestedwhich envision a resilent material or sealing compound as an integralpart in effect of the metal or surfaces, designed and intended to flexunder loads and maintain a fluidtight seal barrier. Thus, it is intendedto prevent moisture from going around the sealing material and attackingthe underlying metal. While these coated parts have shown favorableresults, corrosion has not been completely eliminated because polymerswhich have been employed as the coating material because of their goodresilient characteristics have all been found to be permeable tomoisture. Thus, these prior art materials whether employed wet or as aprecoat fail to maintain the metal in a corrosion-free environmentbecause of moisture penetration through the sealing material.

A unique feature of the invention herein described is that it recognizesthe little known fact that all elastomers and sealants are permeable tomoisture to some degree, and this permeabilty causes many corrosionproblems. Filiform corrosion associated with flexible finishes such asthe aliphatic polyurethanes and its damage to underlying surfaces hasreceived practically no attention. This fact of sealant permeability wasestablished and substantiated in the course of the present invention byspecific tests wherein films of various polymeric material 3 mils inthickness were examined for their water vapor transmissioncharacteristics. Table I summarizes by way of example the amount ofmoisture which diffused through each specified type of film in a 28-daytest period at 86 F. with percent humidty on one side of the membraneand 35 percent humidity on the other side.

TABLE I Water vapor transmission (grams/square meter) The presentinvention builds upon this knowledge in order to produce a moreeffective, more reliable, and a longer wearing protective coatparticularly suited as a sealant for fasteners when installed inaircraft skins but equally effective when employed on other metalsurfaces subject to corrosion. In short, a corrosion-inhibiting coatingand sealing material is presented that has general use and application.To this end, the coating herein proposed is formulated with awater-soluble, reactive inhibiting additive in an elastomeric vehicle,in a sufficient quantity and readily available to prevent corrosion ofthe metal. By incorporating this inhibitor in the coating, a reservoirof protective ions is provided and made available for immediatedissolution in any moisture that may penetrate the elastomeric vehicleor polymer and thereby form a protective solution to prevent corrosionof the underlying metal.

For reasons to become more apparent, in the aircraft application wheretitanium, aluminum, and/or steel and their alloys are used, a chromate(such as magnesium,

calcium, strontium, or barium) is preferred as the inhibiting additive,it being understood that chromates include dichromates or a mixture ofchromates and dichromates. In all cases, the chromate inhibitor isselected according to the particular metal or metals involved and forcompatability with the particular polymer comprising the vehicle. Inpolymeric compounds such as the polysulfides where a cure is effected byoxidation reaction, the chromate can be and sometimes is used as acatalyst. In such case, an excess of some chromates as required hereininterferes with the curing process, and other chromates must be used.

The relative quantity of chromate used varies with the particularpolymer and is critical only to the extent that after the polymer hascured there remains a minimum of about 2 percent by weight of thechromate to neutralize the effect of moisture. Excessive amounts ofchromate tend to make the resulting compound difficult to apply in acontrolled manner or degrade the polymer system. The preferredpercentage is usually on the order of to percent by weight of theultimate or cured compound.

The selection of the particular chromate is limited to one which issufliciently soluble to produce at least a 10- molar concentration ofchromate ions (116 parts of chromate ions per million parts of water)concurrently with the appearance of an aqueous corrosive fluid. This hasbeen determined to be the minimum concentration required to completelysuppress corrosion in crevices and in other areas (such as those under apaint film) where free access to oxygen has been blocked 01f. Inextremely corrosive environments, such as those containing chloride ions(salt water), this concentration must be on the order of 10- molar(11,600 parts of chromate ions per million parts of water).

Preliminary to tests that were run which demonstrated the improvedresults to be obtained by the present corrosion-inhibiting material, aninvestigation of the corrosion process was made. This was accomplishedby the use of test panels subjected to simulated service conditions ofsalt spray, flexing, and exposure to extreme humidity. These panels withrivets installed in the conventional manner were subjected to a total of1440 hours of salt spray exposure, 896 hours of 100 percent humidity,and 260 loading cycles to surface stress of approximately 30,000 psi.

Thereafter, inspection, analysis, and tensile testing of the resultingpanels compared to their pre-test condition indicated that the panelsafter environmental exposure were approximately 5 percent lower instrength than before. It also became apparent that, despite the factthat aircraft skins are protected by cladding or anodizing, the insideof hte fastener holes are susceptible to attack because they are drilledafter the protective coating has been applied.

A further complication is that high strength aluminum alloys, such asfor example 7178-T 6, are not homogeneous but contain appreciableamounts of elements other than aluminum. As a result, they developedanode and cathode areas when in contact with an electrolyte. Underservice conditions, the electrolyte is supplied to the fastener holesand faying surfaces in the form of rain, high humidity, paint strippers,brighteners, and in coastal regions by sea water carried by the wind.

Corrosion around fasteners usually originates in the counter-sink areasof the fastener holes and first attacks the end grains. The metal at thegrain boundaries has a different composition from the metal at theinterior of the grains and is electrochemically anodic relative theretowhereby it is converted to corrosion products when an electrolyte isintroduced. These products occupy a greater volume than the originalmetal and exert tremendous pressures with subsequent expansion withinthe corroded area. The corrosion then follows a laminar pattern alongthe grain boundary and destroys the structural integrity of o the metal.Corrosive fluids in mating joints or faying surfaces attack the metal inthe joint causing it to exfoliate or to stress corrode.

Extensive tests with some 25 inhibitors demonstrated that strontium andmagnesium chromate are extremely eifective in protecting both aluminumand steel from the corrosive effects of water for a period of more thanone year. In these tests, specimens of 7075-T6 aluminum panels coupledwith bare steel fasteners were immersed, some in water and some instrontium chromate solution. After a 12-month period, the bare steelfasteners in the specimens immersed in water were heavily rusted, and atthe bottom of the jar was a layer of ferric hydroxide. On the otherhand, the specimens immersed in the strontium chromate solution were inperfect condition and even the steel fasteners were free of oxidationafter exposure of 18 months.

From these and similar tests, it was concluded that the chromateinhibitors provide complete corrosion protection in water at lowerconcentrations than other inhibitors and that both strontium andmagnesium are effective in substantially lower concentrations than areother chromate inhibitors. Potentiostatic studies were then conductedwith each of these inhibitors to select the better one. For thispurpose, a Model 410 Anatrol Research Potentiostat was used toinvestigate the behavior of 7075-T6 aluminum alloy in 3 percent sodiumchloride solution with and without inhibitors present. The Potentiostatmaintained the test specimen at a desired control potential with respectto a calomel reference electrode. The calomel electrode was immersed insaturated potassium chloride solution and was connected to the test cellby means of a salt bridge. The control potential was varied from -3 to+3 volts during each scan and the current voltage relationships wereobtained by simultaneously plotting the current and the controlpotential on a Sanburn Dual Channel Recorder.

The current flow at a given voltage is a function of the type of testspecimen, concentration and type of electrolyte, and type andconcentration of inhibitor. The inhibitors were evaluated by comparingthe otentiostatic polarization curves. On the basis of the results ofthe potentiostatic studies, magnesium chromate inhibitor was selected tobe added to polyurethane and to Buna N, and these combinations wereevaluated in fastener coating programs.

In one particular program that demonstrated the performance of fastenersin installation tests, titanium fasteners of the rivet as well asnut-and-bolt type were used. These fasteners were precoated by dippingthem into a solution which contained 66.6 grams black Goodrich 412polyurethane, 33.3 grams orange Goodrich 412 polyurethane, 10 grams ofcolloidal magnesium chromate, and 40 millimeters xylene. The fastenerswere then placed upright with the head down on absorbent paper whilethey were draining. They were air dried for 16 hours and given a finalcure by heating to F. for 4 hours.

The fasteners thus coated were then installed in panels which weresubjected to fatigue and corrosion tests. The coated fasteners becauseof the 0.5 to 1.5 mil thickness of inhibited polyurethane coating,extended above the top level of the specimen placed to a height of about.280 inch. This extension was reduced to .150 inch by gently tapping theend of each of the rivets. A nut was put on the threaded end of each ofthe other fasteners, and it was torqued into place by a normalrotational force. The interference of the fasteners with theirrespective holes caused some of the coating from the shank to be pushedunder the countersink.

When the fasteners were finally installed into place, i.e., home, theexcess polymer extruded around the side of each fastener head; and inthe case of the threaded fasteners, the extruded polymer rolled in arelatively clean, thin cylinder along the shank as it was being torquedby the nut. In any case, the excess material was readily removed fromthe top of the specimen plate by gently brushing the surface. This is aparticularly desirable feature because it eliminates the messy cleanuprequired by encapsulated and wet installed fasteners.

Moreover, in the case of the threaded fasteners, as the individual nutswere tightened and the nuts moved along the bolt threads, they cleanedthe excess coating out of the thread and moved it to the top of thethread adjacent to the plate. The excess material was then compressed toform a compression fillet which formed a seal at the under side of thefastener hole in contact with the adjacent edge of the plate.

The completed assemblies were overcoated with a conventional aircraftpaint system prior to testing. The speciments were subjected to 100hours of salt-fog exposure and 20,000 cycles of fatigue loading at 20Ks.i. Fastener heads were scribed to simulate fatigue cracking of thecoating as would occur under normal service conditions, and thespecimens were given an additional 500 hours of salt-fog exposure.Section specimens showed no corrosion of fastener holes below thescribed line. These fasteners coated with corrosion-inhibitedpolyurethane gave superior corrosion protection when compared withfasteners installed by other methods, including the conventionalwet-riveting practice.

When the results of the above experiments were related to present daymanufacture and production practice it was concluded that the cure rateof polyurethane compounds in the type of system contemplated leftsomething to be desired from a practical standpoint. Thus, the worklife, i.e., the length of time the coating and sealing material can bemade to remain mastic or flowable was much less than desired. Moreover,control of the release of chromate ions is extremely difficult if at allpossible in an efficient manner with polyurethanes.

Alternate polymers were, therefore, considered in an effort to achieve abetter cure rate and also greater control of inhibitor release. Theprobabilities of success seemed remote because polymers having thedesired resiliency cure by an oxidation reaction and the use of anoxidizing catalyst like the chromates was felt would cause a prematurecure. Also, it was felt that the relatively large percentage ofcatalyst, sufficient to leave on the order of to percent by weight inthe compound after cure as envisioned by the invention, would effectexcessive cross linking causing the compound to overcure and therebybecome hard and brittle.

Nevertheless, the experiments were conducted because of the magnitude ofthe problem and with the hope that by carefully controlling the catalystto polymer ratio some small gain in corrosion prevention might bepossible. Thus, by the use of minute percentages of oxidizing additivesto the elastomer, some optimum point of compromise might be reachedbetween the cure time and physical properties, especially of theultimate coating and its ability to prevent corrosion due to moisturepermeation.

After some preliminary experiments, a test was run in which 10 grams ofmagnesium chromate were added to 100 grams of polysulfide sealant whichalready contained the optimum amount of catalyst (approximately 2grams). Fasteners were coated with the resulting mix and were permittedto cure overnight at room temperature. After 16 hours, the sealant wascured and, contrary to expectations, was still tough and resilient withno evidence of polymer degradation. Because of these findings, andothers such as characteristics of polysulfides which permit their curein a more manangeable manner and at room ('ambient) temperatures,polysulfide became the preferred polymer system for producing theinhibitive coatings.

In retrospect, the unexpected performance of polysulfide with the undulylarge chromate additive has been attributed to the fact-thatpolysulfides have a limited number of active sites necessary for crosslinking. There fore, excessive cross linking cannot occur. Also, it hasbeen found that polysulfides lend themselves well to the chromateadditives and the release rate of chromate ions may be readilycontrolled by selection of the chromate or mixtures of differentchromates.

Magnesium chromate was selected as the preferred inhibitor because it isreadily soluble in water and because the previous potentiostaticpolarization test results demonstrated that it is more effective incorrosion prevention for aluminum than is sodium, potassium or strontiumchromate. This has been attributed to the fact that the magnesium ionsprovide cathodic inhibition and the chromate ions provide anodicinhibition so a double pro tection process occurs when magnesiumchromate is used. In contrast, sodium and potassium chromate generatesodium hydroxide (NaOH) and potassium hydroxide (KOH) respectively atthe cathodic sites. In the case of aluminum metal, this strong alkalineconcentration causes accelerated corrosion. Strontium chromate was foundto be an excellent inhibitor in water solution but, when incorporated inan elastomeric polymer, its limited solubility makes it diflicult forwater in contact with the polymer to rapidly reach a Molar chromate ionconcentration of 10- From all test results, it became apparent, amongother things, that exfoliation corrosion of fastener holes is caused bygalvanic action when electrolytes penetrate the space between thefasteners and the fastener holes. The sealing of the fasteners in theholes with zinc chromate primer or elastomeric sealants provides goodprotection. Even greater protection is obtained when an elastomericpolymer, particularly a polysulfide, is employed which incorporates achromate inhibitor in predetermined quantities so as to include anexcess of chromate ions in the fully cured polymer to combine with anymoisture seepage and thereby produce a corrosion-free environment.

Comparing rivets coated as herein proposed with those following priorpractices, substantial installation time required for the Wet rivetingis avoided. Moreover, mixing solid inhibitor with resilient material asherein proposed is at least considerably easier and less expensive thanthe encapsulated inhibitor process, and the storage stability is farsuperior. The inclusion of an inhibitor as herein proposed can beaccomplished without interference with or degradation of the resilientmaterial and the polymer cure can be accomplished without heat, i.e., atambient temperauture. At the same time, the inhibitor is available todissolve in any corrosion fluids when and if they permeate the sealantmaterial so as to form a protective solution and prevent attack of theunderlying metal surfaces thereby.

It is to be understood that the foregoing disclosure is in terms ofspecifics contemplated by the instant invention for purposes ofclarification and a thorough understanding thereof. The elastomericcorrosion-inhibiting coating and sealing material proposed herein isequally effective when employed as a precoat to fasteners and othermetal parts or when applied in situ, as for example in wet riveting. Theappended claims are intended to cover all variations and modificationswhich will be apparent to those skilledin the art and which fairly fallwithin the true spirit and scope of the invention.

What is claimed is:

1. A coating and sealing composition for application to metallicsurfaces comprising a resilient, curable elastomeric polysulfide polymerand a corrosion-inhibiting, soluble chromate compound in a quantitysufiicient to assure an available amount thereof of at least about twopercent by Weight of the polymer when cured and to yield at least abouta 10- molar concentration of chromate ions concurrently with theappearance of aqueous corrosive fluids.

2. In a coating and sealing composition for application to metallicsurfaces which includes a resilient, curable elastomeric polysulfidepolymer, the improvement in combination therewith comprising acorrosion-inhibiting,

water-soluble chromate compound in a quantity sufficient to assure anavailable amount thereof of at least about two percent by weight of thepolymer when cured and to yield at least about a 10 molar concentrationof chr0- mate ions concurrently with the appearance of aqueous corrosivefluids.

3. The composition of claim 1 wherein said chromate is magnesiumchromate.

4. The composition of claim 1 wherein said available amount of chromateadditive is on the order of five to ten percent by weight of the curedpolymer.

5. The composition of claim 1 wherein said chromate additive comprisesan admixture of different chromates.

6. The composition of claim 1 wherein said chromate additive is in aquantity capable of yielding about 10- 15 molar concentration ofchromate ions.

References Cited UNITED STATES PATENTS 11/1947 Sloane 106--14 5/1962Sampson 10614 OTHER REFERENCES Chemical Abstracts, vol. 60: 8890g, 1964.Burns et al.: Protective Coatings for Metals, Rhein- 10 hold Publishing-Co., 1967, pp. 414-417 relied upon.

U.S. Cl. X.R. 106-14

